They are harassing me

In March we tried to change our email address with Telkom. We phoned the Tel number 1023, and they said they will send us a form to complete and send back to them. My secretary received the form and brought to me to sign, which I did. The next thing i received a account for R14 400 for advertising services rendered by telekom1023 for 12 months. They said the form I completed and signed was a contract? I have explained to numerous people what happened but then someone else just phones again from debt collectors. Help!